The chemical properties of the dipyridylium cations are very important to our understanding of several aspects of the photosynthetic process. These organic cations act as herbicides, as artificial electron acceptors in the electron transfer pathway of chloroplasts, as agents for the incorporation of magnesium ion into porphyrin-type rings to form chlorophy11 analogs, as redox indicators, as catalysts for the photooxidation of chlorophylls, and as component reagents in oxygen analysis apparati. It is our view that there are significant interrelationships between the several actios of these cations; in some cases relationships are obvious whereas in other instances it is not apparent. Further, the behaviors are not fully understood. In order to contribute to a better understanding of the chemisty of the cations, their radicals and photosynthesis, we plan to study them using techniques as follows: 1) Polarographic reduction of the cations to their radical cations coupled with electron spin resonance and spectrophotometric analyses; 2) chemical reduction of the viologens, again wit esr and spectral analyses; 3) the influence of media on the behavior of the cations and radical cations; 4) stoichiometries and kinetics of the oxidation of the radical cations by oxygen, superoxide, hydrogen peroxide and other peroxides; 5) interactions of radical cations and chlorophylls; 6) the behaviors of the cations as herbicides; and 7) studies of magnesium insertion into porphyrins as catalyzed by radical cations.